Hi Metlclub,
First of all, a good way to approach this question is to think of airflow like waterflow. They are not exactly the same, but there are similarities between airflow and waterflow. For ideal water speed, you want the flow to go in a single direction, with one inlet and one outlet. Right now, your air is kind of swirling around, like two rivers colliding at different directions. The ideal airflow would be one side intake, one side exhaust. However, for OEM stock cases, there are limitations and only so much you can tweak this.
So the question for you is not how to achieve ideal airflow, but how to achieve the best possible for your case. Then again, you also have to consider the trade-off between dBA levels and air flow volume (CFM, cubic feet per minute). To achieve the maximum amount of air cooling on your case, it looks like you should put one high CFM, high RPM 120mm fan blowing right on, or as close as possible, to your CPU/heatsink assembly, like THIS fan
http://www.newegg.com/Product/Product.aspx?Item=N82E16835213001.
That would be your inlet direction. Make sure this inlet fan is not obstructed and has plenty of cool air to draw from, and turn that sucker on. That will blast a tunnel of cool air onto the CPU and push that hot air away. Then I would think you would want to have 3 exhaust fans on the other 3 openings, running slower and more quietly. Those guys don't have to move a lot of air, their just making sure that the air blown by fan #1 has a place to exit. So for the exhaust fans, you could get 3 lower RPM, cheaper fans like this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835103069. However, I would still stay at 120mm, you don't want to have tiny outlet holes.
If I was you and I wanted to get the maximum air cooling with your case, this is how I would approach it. Feel free to approach it differently if you want. Just remember, there's only so much case cooling you can do with bigger OEM cases and pre-set mounting spots.
Good luck.
